Conventionally, there have been proposals for wavelength-type velocity measuring devices that use the self-coupling effect of a semiconductor laser (See Japanese Unexamined Patent Application Publication 2006-313080). A structure for this velocity measuring device is given in FIG. 17. The velocity measuring device of FIG. 17 includes a semiconductor laser 201 for emitting a laser beam at an object 210; a photodiode 202 for converting the optical output of the semiconductor laser 201 to an electric signal; a lens 203 for focusing the beam from the semiconductor laser 201 to illuminate the object 210, and to focus the beam returned from the object 210 to cause it to be incident into the semiconductor laser 201; a laser driver 204 for repetitively switching the semiconductor laser 201 between a first emitting interval wherein the oscillating wavelength increases continuously and a second emitting interval wherein the oscillating wavelength decreases continuously; a current-voltage converting amplifying portion 205 for converting and amplifying the output current from the photodiode 202 into a voltage; a signal extracting circuit 206 for deriving a signal from the output voltage of the current-voltage converting amplifying portion 205; a counting device 207 for counting the number of mode-hop pulses (hereinafter termed “MHPs”), which is an interference waveform due to the self-coupling effect, included in the output voltage of the signal extracting circuit 206; a calculating device 208 for calculating the speed of the object 210; and a display device 209 for displaying the calculation result by the calculating device 208.
The laser driver 204 provides, as a driving current to the semiconductor laser 201, a triangle wave driving current that repetitively increases and decreases at a constant rate of change in respect to time. As a result, the semiconductor laser 201 is driven so as to repetitively alternate between a first emitting interval wherein the oscillating wavelength continuously increases at a constant rate of change, and a second emitting interval wherein the oscillating wavelength is continuously reduced at a constant rate of change. FIG. 18 is a diagram illustrating the changes in the oscillating wavelength of the semiconductor laser 201 over time. In FIG. 18: P1 is the first emitting interval; P2 is the second emitting interval; λa is the minimum value for the oscillating wavelength in each interval; λb is the maximum value for the oscillating wavelength in each interval; and Tcar is the period of the triangle wave.
The beam that is emitted from the semiconductor laser 201 is focused by the lens 203 to be incident on the object 210. The beam that is reflected from the object 210 is focused by the lens 203 to be incident into the semiconductor laser 201. The photodiode 202 converts the output of the semiconductor laser 201 into an electric current. The current-voltage converting/amplifying portion 205 converts the output current from the photodiode 202 into a voltage, and then amplifies that voltage, and the signal extracting circuit 206 removes the emitted waveform (the carrier wave) of the semiconductor laser 201 from the output voltage of the current-voltage converting/amplifying portion 205. The number of MHPs included in the output voltage of the signal extracting circuit 206 is counted by the signal counting device 207 for the first emitting interval P1 and for the second emitting interval P2. The calculating device 208 calculates a physical quantity, such as the velocity of the object 210 based on the minimum oscillating wavelength λa and the maximum oscillating wavelength λb of the semiconductor laser 1 and the number of MHPs in the first emitting interval P1 and the number of MHPs in the second emitting interval P2.
While in the conventional velocity measuring device that uses a triangular carrier wave it is possible to perform calculations separating the interference signal into distance and speed, when compared to a common interferometer that uses a sawtooth carrier wave, there is a problem in that the range of velocity measurement is narrow. This problem area is described below.
The number of MHPs measured by the measuring device is a linear sum of a term N0, which is proportional to the distance L from the object, and a term A·V (where A is a coefficient) that is proportional to the speed of the object, as in the following equation:N=|N0±A·V|  (1)
Here the velocity V is positive in the direction that is approaching the semiconductor laser. N=|N0+A·V| during a first emitting interval wherein the oscillating wavelength of the semiconductor laser is increasing, and N=|N0−A·V| during a second emitting interval wherein the oscillating wavelength is decreasing. Moreover, when the relationship in Equation (1) is expressed in terms of the frequency fsig of the MHPs, the result is the following equation:fsig=|f0±a·V|  (2)
f0 is a term that is proportional to the distance L to the object, and a is a coefficient. The relationship between the MHP frequency fsig and the velocity V of the object is illustrated in FIG. 19. In FIG. 19, fu indicates the change in the frequency fig in the first emitting interval wherein the oscillating wavelength of the semiconductor laser is increasing, and fd indicates the change in the frequency fig in the second emitting interval wherein the oscillating wavelength is decreasing. fmax is the maximum frequency of the MHPs that can be extracted by the signal extracting circuit 206, and fmin is the minimum frequency of the MHPs that can be extracted by the signal extracting circuit 206. The difference between fmax and fmin is the circuit bandwidth.
As described above, in a velocity measuring device that uses the triangular carrier wave, the signs of the coefficients relating to the velocities V of the object are different during the first emitting interval and the second emitting interval. Because of this, as illustrated in FIG. 20, wherein the semiconductor laser is caused to oscillate in a sawtooth wave, as illustrated in FIG. 20, the circuit bandwidth they can be used in counting the MHPs is half as large, and as a result, the circuit bandwidth they can be used in the velocity measurement is half as large. Note that the problem set forth above is not limited to self coupling-type velocity measuring devices, but occurs similarly in a velocity measuring device that uses a triangular carrier wave.
On the other hand, in a velocity measuring device that uses the sawtooth carrier wave, illustrated in FIG. 20, it is possible to double the range of measurement of velocities when compared to the velocity measuring device that uses the triangular carrier wave. However, in the MHPs counted by the measuring device, it is not possible to separate those MHPs due to the distance from the object from the MHPs due to the speed of the object. The result is a problem in that there will be error in the speed that is calculated by the calculating device.
The present invention is to solve the problem set forth above, and the object is to provide a velocity measuring device and method able to increase the range of measurement of velocity while maintaining the benefits of a velocity measuring device that uses a triangular carrier wave.